1.00/1.001
Introduction to Computers and Engineering Problem Solving
Recitation 4
Static
Array & ArrayList
Spring 2012
1
Quiz 1
• March 9 (Friday)
• 3:05pm-4:25pm
• Review session:
– Wed. March 7, 7pm - 9pm
• LA office hour: Thu 7:00-9:00pm
• Open notes/book, no electronic devices
2
Today’s Recitation
• Keywords: Static
• Array and ArrayList
3
Static
• Static members:
– not associated with any particular instance of the class—
one copy shared by all instances
– accessible to both static and non-static methods
• Static Methods:
– may only access static members, not instance members
– should be called using Classname.methodName()
4
Static Members
public class Number {
private int num;
...
}
Number object 1
Number object 2
num = ...
num = ...
5
Number object 3
num = ...
Static Members
public class Number {
private static int num;
...
}
Number object 1
Number object 2
num = ...
num = ...
6
Number object 3
num = ...
Static Methods
public class Number {
private static int num;
private int num2;
public int sum() {
return num + num2;
}
Is this ok?
...
}
7
When to Use Static Methods
• When no access to any instance field is required. Usually one
of two scenarios:
– The method takes in all the information it needs as
arguments:
Math.pow(double base, double exp)
– Or, the method needs access to only static variables.
– Usually you can think of these methods as taking in some
information and performing a service for you
– Typically, they do not alter the state of the class, as they do
not have access to instance variables
8
Exercise 1: Static Members
• Write a class Ticket that
– Keeps track of how many tickets there are
– Assigns a unique ticket number to each ticket, starting with
100, 101, etc.
– Has a method to return the number of tickets
– Has a method to return the ticket number
• Write a class TicketTest that creates some
Ticket objects and then prints out how many
were created.
9
Keywords Summary
public / private
Control access to data members and methods. Public data members can be
accessed outside the class, and public methods can be invoked outside the class.
Private data members and methods are not visible outside the class
static
Each instance (object) of a class has its own copy of each non-static data member,
but there is only one copy of each static data members, shared by all instances
(objects) of the class.
A static method can only use the static data members of the class and its input
parameters. Static methods are invoked on the class name, not on any object
name.
void
Apart from constructors, every method has a declared return type. If a method does
not return anything, its return type must be void.
final
The value of a final data member cannot be modified after it has been initialized.
10
Keywords Summary
public class UGrad{
What is the function of each keyword in this class?
private double gpa;
public static final int MAX_GPA = 5;
public double getGPA() {
return gpa;
}
public void printGPA() {
System.out.println("GPA: " + gpa + " / " + MAX_GPA);
}
public static int getMaxGpa() {
return MAX_GPA;
}
}
11
Array
vs.
•
•
•
•
ArrayList
• Size varies as data is
added/removed
• Accessed with
z.get()
• Object with no data
members
• Has lots of methods:
e.g., z.add(),
z.size()
• More flexible
Size fixed at creation
Accessed with z[]
Object with no methods
One public data
member: z.length
• Slightly faster
12
Array and ArrayList
• Setting and accessing data is different:
int[] array = new int[3];
for(int i = 0; i < array.length ; i++) {
array[i] = i;
}
What does array contain?
ArrayList<Integer> arrayList = new ArrayList<Integer>();
for(int i = array.length-1; i > -1 ; i--) {
arrayList.add(array[i]);
}
What does arrayList contain?
13
Array or ArrayList?
• Which would you use for the following
problems: an Array or ArrayList?
– Write a method that returns all the primes
between 2 and a specified number
– Write a method that returns a specified number of
random numbers
14
ArrayList Exercise
• Write a method findPrimes(int n) that
returns an ArrayList<Integer> of all the
primes between 2 and n
• Start by putting in all the numbers, then remove the
ones that are multiples of each other in the ArrayList
• Some code is provided
15
ArrayList Exercise
public static ArrayList<Integer> findPrimes(int n) {
//Declare the ArrayList
for (int i=2; i<=n; i++)
//Put i in the ArrayList
int i = 0;
while //condition to make sure i is a valid index
int j = i + 1;
while //condition to make sure j is a valid index
if //(element j)%(element i)==0
//remove the proper number from the list
else
j++;
}
i++;
}
//return your ArrayList
}
16
Array Exercise
• Write a method makeRandom(int n) that
returns an array of n random numbers
• Some code is provided
17
Array Exercise
private static Random r = new Random();
public static int[] makeRandom(int n) {
//Create your array, then assign random
//numbers using r.nextInt();
}
18
Self assessment, lectures 1-11
Skill
Below
Expected
Above
Data types
Types misused.
Static not used
Ints and reals used
distinguishably.
Static used
correctly
Int, long, double,
boolean, and static
used purposefully
Variables
Numbers and
variables used
without distinction
Variables used for
most quantities
Variables easy to
read. Naming is
consistent and
accurate
Expressions
Complex
expressions not
defined correctly;
simple expressions
ok
Complex
expressions
organized by
parentheses and
use of variables
Complex
expressions
structured for
increased clarify
Loop constructs
Successfully used
Clear and
understandable
Appropriate choice
of for, while, dowhile
19
Self assessment, lectures 1-11
Skill
Below
Expected
Above
Methods
Methods not clearly
defined, or use
poor arguments or
return values.
Wrong use of static
Methods defined
but overall
structure not
always clear. Static
used correctly.
Methods organized,
named clearly,
perform clear
behaviors. Static
used appropriately
Method arguments
Few or no
arguments used
Appropriate
arguments used
Arguments versus
data members
clearly designed
Variable scope
Local variable scope Most variables have All variables
inconsistent, often appropriate scope
defined just before
too large
use and go out of
scope after use
20
Self assessment, lectures 1-11
Skill
Below
Expected
Above
Comments
Some critical
comments missing,
some not clear
Comments explain
basic code
Comments make
code selfexplanatory
Indentation
No indentation
used
Some indentation
used but not
consistent
All code properly
indented
21
Homework 4: Scrabble
Z
E
T
R
L
C
Dictionary
H
twoLetterWords =
What are all the two and three
letter words that can be made
with your hand of 7 letters?
“ET”
ArrayList
of Strings
“ETH”
“ER”
“AA”
“AB”
“AD”
…
threeLetterWords =
…
array of
Strings
22
0 “AAH”
“AAL”
“AAS”
…
1 “BAA”
“BAD”
“BAG”
…
2 “CAB”
“CAD” “CAM”
3 “DAB”
“DAD”
“DAG”
4 “EAR”
“EAT”
“EAU”
…
…
…
…
…
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Spring 2012
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